Heat pipes are highly efficient devices for transferring large quantities of heat from a heat source to an area where the heat can be dissipated. A heat pipe generally consists of a vacuum tight envelope, a wick structure and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, typically just enough to saturate the wick. The atmosphere inside the heat pipe is set by an equilibrium of liquid and vapor. As heat enters at the evaporator, this equilibrium is upset generating vapor at a slightly higher pressure. This higher pressure vapor travels to the condenser end where the slightly lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure. This continuous cycle transfers large quantities of heat with very low thermal gradients. A heat pipe's operation is passive being driven only by the heat that is transferred.
Heat pipes are currently used for a variety of applications including lasers, nuclear energy, dehumidification and air conditioning, thermal control in spacecrafts, cooling of electronics systems and cryogenics. Heat pipes can be designed to operate over a broad range of temperatures from cryogenic applications (<−243° C.) to high temperature applications (>2000° C.). The material for the heat pipe container, wick structure and working fluid are selected based on the application for which the heat pipe will be used.
Heat pipes are generally designed to perform within a particular operating temperature range, which is dependent on the application. When operating within this range, a heat pipe will provide highly reliable heat transfer for years. However, operation outside of this range causes degradation or even failure. Such a limitation is a disadvantage in applications where there may be peak loads which can cause a sudden temperature rise outside of the operating range of the heat pipe. Not only can this cause failure of the heat pipe, but also may cause damage to the system which is being cooled.
Temperature control devices using phase change materials (PCMs) have been employed in a variety of temperature stabilization applications including automotive, electronics and clothing applications. Often, PCMs, e.g., wax, are encapsulated in a durable, thermally conductive shell. PCMs provide a temperature load-leveling capability via the latent heat effect. PCMs store or release heat as they change phase between a liquid and solid state or, in the case of solid—solid PCMs, as they undergo reversible crystal structure transitions. The relatively high thermal capacity of PCMs make them advantageous for temperature control in high heat generating systems and for systems prone to transient peak loads. Electronics systems are one such system where PCMs may be particularly advantageous.
The increasing miniaturization of electronic components has made heat transfer a critical design concern as these systems create very high heat fluxes. In order for electronic devices to perform correctly and reliably, suitable operating temperatures must be maintained and temperature variations must be minimized. Due to the increasingly high heat generated from these systems, and their proneness for transient peak loads, common heat transfer technologies such as heat sinks, cold plates, direct impingement cooling systems and conventional heat pipes are approaching their heat transfer limits.
The use of PCMs in heat transfer devices is known in the art. For example, in U.S. Pat. No. 5,224,356 to Colvin et al., a method is disclosed whereby a plurality of microcapsules in the form of a powder are placed in contact with an object to be cooled. The microcapsules have a shell and contain an enhanced thermal energy absorbing material. The absorbing material may be a phase change material.
U.S. Pat. No. 5,007,478 to Sengupta, discloses a heat sink device adjacent to an article to be thermally controlled. The heat sink defines a chamber which contains a slurry of micro-encapsulated PCMs.
U.S. Pat. No. 5,831,831 to Freeland, discloses a bonding material/phase change material system for electronic device heat burst dissipation. The system comprises a phase change material disposed on a substrate and encircled by a bonding material. An electronic device having a heatspreader portion is positioned atop the phase change material and bonding material.
U.S. Pat. No. 5,555,932 to Dudley, discloses a heat shield for an automotive vehicle. The heat shield utilizes a phase change material to absorb excess heat generated by a heat source within the vehicle. The heat shield insulates a component adjacent to the heat source and prevents the transmission of heat to the component.
U.S. Pat. No. 4,911,232 to Colvin et al., discloses a method of obtaining enhanced heat transfer in a closed loop thermodynamic system. The system includes a two-component heat transfer fluid comprising a carrier fluid and a plurality of discrete reversible latent energy transition material particles. The fluid slurry is circulated about the loop and the loop is tuned so that a minimum temperature differential exists between the thermal source and sink in order to maximize the latent heat transport by adjustment of the heat transfer fluid flow rate, the rate of thermal energy input into the heat transfer fluid and the rate of cooling of heat transfer fluid. This method has the disadvantage of needing an outside energy source to pump the heat transferring slurry.
The above disclosures all relate to thermal regulating systems wherein the PCMs are contained within a structure which is substantially completely adjacent to the heat source. Thus, the PCMs release the absorbed heat, as well as absorb the heat, at a location proximate the heat source. They lack the advantage of having the heat released at a location distant from the heat source.